I consider myself an old-school gamer, even though the version of D&D my group currently plays is relatively recent (3.5e). Why? Because I am old. I've been playing RPGs for roughly 33 years and, I have to admit, my style preferences were probably set a long time ago. Those preferences were largely informed by the writings of one man, Gary Gygax. Those original first edition rule-books and modules written by Gygax had a tone and style that felt mysterious, cryptic, and adult to me. It was a style I wanted to emulate, and to a degree, still do. When TSR kicked Gygax out of his own company and started pumping out second edition D&D I quit buying their products. I only picked up D&D again with the third edition because enough time had passed and I wanted to see how Wizards of the Coast had overhauled the system. Sure, a lot had changed in the rules between 1e AD&D and 3e D&D, and a case can be made that a game's rules set the overall tone of the game, but I actually found a lot in 3e that was reminiscent of the old, original game I enjoyed, just with updated game rules theory.
And then there is the fact I actually don't buy any new games. It is true that I bought 3e D&D when it came out, but I haven't bought any new RPGs since. (My set of 3.5e books was a gift from a player who wanted our group to have the "refined" rules. Cool by me... for free!) Since the time we first bought the 1e AD&D books as kids I had no intention of buying any more. Why should I? I had everything there I needed for a lifetime of gaming. And anything that felt broken could always be fixed with house-rules. My blood boils at the concept of game companies pumping out new editions of their game simply because they want to feel a renewed flush of profits. Fuck that.
The games I have sitting on my shelf waiting for the right mood to strike have been sitting there for decades. Call of Cthulhu, Villains & Vigilantes, Top Secret, Stormbringer, and the mighty Traveller. I even have a set of Dangerous Journeys that I am dying to play at least once in my life. All these games rate differently on the scale of complexity, but they all share a couple of things in common... they are all old, and they all have an awesome atmosphere and style to them. For me, that is "old-school".